Method for producing nanocomposite magnet using atomizing method

ABSTRACT

A rare-earth alloy powder is obtained by rapidly cooling a melt of an alloy by an atomization process. The alloy has a composition represented by (Fe 1-m T m ) 100-x-y-z Q x R y Ti z M n , where T is at least one of Co and Ni, Q is at least one of B and C, R is at least one of the rare-earth metal elements and yttrium, and M is at least one of Nb, Zr, Mo, Ta and Hf. The mole fractions x, y, z, m and n satisfy 10 at %&lt;x≦25 at %, 6 at %≦y&lt;10 at %, 0.1 at %≦z≦12 at %, 0≦m≦0.5, and 0 at %≦n≦10 at %, respectively. By adding Ti to the alloy, the nucleation and growth of α-Fe during the rapid quenching process can be minimized.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of making a nanocompositemagnet powder of an iron-based rare-earth alloy by an atomizationprocess.

2. Description of the Related Art

Nd—Fe—B based iron-based rare-earth magnet alloys are currently usedextensively in sintered magnets and bonded magnets. These magnets areproduced in different ways. More specifically, a sintered magnet is madeby pulverizing a magnet alloy, obtained by an ingot casting process, astrip casting process or any other process, compacting the pulverizedpowder, and then sintering the powder compact. On the other hand, abonded magnet is made by rapidly cooling and solidifying a molten alloyby a melt spinning process, for example, pulverizing the resultantrapidly solidified alloy into a powder, compounding the powder with aresin, and then molding the mixture into a desired shape. In thismanner, no matter whether the magnet to be produced is a sintered magnetor a bonded magnet, its magnet powder is always obtained by pulverizingthe material magnet alloy. That is to say, the manufacturing process ofmagnets normally includes a pulverizing process step as an indispensableprocess step.

However, there are some methods of making a magnet powder withoutperforming such a pulverizing process step. A gas atomization process isone of such methods that are known in the art. In the gas atomizationprocess, a melt of an alloy is sprayed through a nozzle, for example,into an inert gas, and is made to collide against the gas, therebycooling the melt droplets. In this manner, spherical particles withparticle sizes on the order of several tens of micrometers can be formeddirectly. In the gas atomization process, the droplets of the moltenmetal are solidified while being carried in the gas-flow, thus formingsubstantially spherical particles. The powder obtained by the gasatomization process (i.e., an atomized powder) has preferred shapes andparticle sizes as a magnetic powder to make a bonded magnet.

If the atomization process is used to produce a bonded magnet, then theatomized powder can be used as it is as a magnet powder for a bondedmagnet. Thus, no mechanical pulverizing process step is needed and themanufacturing cost can be reduced significantly. It should be noted,however, that the particle sizes of such an atomized powder are greaterthan those of a magnet powder to make a sintered magnet. Accordingly, itis difficult to use the atomized powder as it is as a magnet powder fora sintered magnet.

In a rapidly solidified rare-earth alloy magnet powder, which iscurrently used extensively to make a bonded magnet, an Nd₂Fe₁₄B basedcompound phase with a crystal grain size of about 20 nm to about 200 nmis finely dispersed in the powder particles. Such a nanocrystallinestructure is formed by rapidly cooling a molten alloy with a particularcomposition by a melt spinning process, for example, to make anamorphous alloy thin strip and then thermally treating and crystallizingthe amorphous alloy thin strip.

Meanwhile, high-performance magnets, having a quite different metalstructure from that of the rapidly solidified magnet described above,are also under vigorous development. A typical example of those magnetsis a composite magnet called a “nanocomposite magnet (exchange springmagnet)”. The nanocomposite magnet has a metal structure in which hardand soft magnetic phases are finely dispersed and in which therespective constituent phases are magnetically coupled together throughexchange interactions. The respective constituent phases of thenanocomposite magnet have nanometer-scale sizes and its nanocrystallinestructure, defined by the sizes and dispersiveness of the respectiveconstituent phases, has significant effects on its magnet performance.

Among those nanocomposite magnets, a magnet in which an Nd₂Fe₁₄B basedcompound phase (i.e., hard magnetic phase) and α-Fe, iron-based borideand other soft magnetic phases are distributed in the same metalstructure, attracts particularly much attention. In the prior art, sucha nanocomposite magnet is also made by rapidly cooling a molten alloy bythe melt spinning process and then thermally treating and crystallizingthe rapidly solidified alloy. If the powder of such a rapidly solidifiedmagnet could be made by the gas atomization process, then no otherpulverizing process step would be needed and the manufacturing costcould be reduced significantly.

Actually, however, it is very difficult to make the rapidly solidifiedmagnet powder by the atomization process. This is because themelt-quenching rate by the atomization process is lower than that by themelt spinning process by as much as one to two orders of magnitude.Thus, a sufficiently amorphized alloy structure cannot be formed by theconventional atomization process.

As for non-nanocomposite rapidly solidified magnets (i.e., rapidlysolidified magnets including only the Nd₂Fe₁₄B based compound phase), amethod of making an amorphous alloy producible even by the atomizationprocess with such a low quenching rate by increasing the amorphousforming ability (i.e., quenchability) of the alloy with the addition ofTiC, for example, was proposed.

However, as for an α-Fe/R₂Fe₁₄B based nanocomposite magnet, it isdifficult to produce an actually usable, high-performance magnet by theatomization process. The reason is that if the quenching rate is as lowas in the atomization process, the soft magnetic α-Fe phase easilynucleates and grows earlier than the R₂Fe₁₄B phase and increases itssize so much that the exchange interactions among the respectiveconstituent phases weaken and that the magnetic properties of theresultant nanocomposite magnet deteriorate significantly.

In order to overcome the problems described above, a primary object ofthe present invention is to make a powder of a nanocomposite magnet withexcellent magnetic properties producible by the atomization process.

SUMMARY OF THE INVENTION

A nanocomposite magnet powder of an iron-based rare-earth alloyaccording to the present invention has a composition represented by thegeneral formula: (Fe_(1-m)T_(m))_(100-x-y-z)Q_(x)R_(y)Ti_(z)M_(n) (whereT is at least one element selected from the group consisting of Co andNi; Q is at least one element selected from the group consisting of Band C; R is at least one element selected from the group consisting ofthe rare-earth metal elements and yttrium; and M is at least one elementselected from the group consisting of Nb, Zr, Mo, Ta and Hf). The molefractions x, y, z, m and n satisfy the inequalities of: 10 at %<x≦25 at%; 6 at %≦y<10 at %; 0.1 at %≦z≦12 at %; 0≦m≦0.5; and 0 at %≦n≦10 at %,respectively. The nanocomposite magnet powder includes at least twotypes of ferromagnetic crystalline phases, in which a hard magneticphase has an average grain size of 10 nm to 200 nm and a soft magneticphase has an average grain size of 1 nm to 100 nm.

In one preferred embodiment, an R₂Fe₁₄B type compound phase, a boridephase and an α-Fe phase are present in the same metal structure.

In another preferred embodiment, the α-Fe and boride phases have anaverage crystal grain size of 1 nm to 50 nm, which is smaller than theaverage crystal grain size of the R₂Fe₁₄B type compound phase, and arepresent on a grain boundary or sub-boundary of the R₂Fe₁₄B type compoundphase.

In another preferred embodiment, the boride phase includes aferromagnetic iron-based boride.

In another preferred embodiment, the iron-based boride includes Fe₃Band/or Fe₂₃B₆.

In another preferred embodiment, the magnet powder has a mean particlesize of 1 μm to 100 μm.

In another preferred embodiment, the magnet powder exhibits hardmagnetic properties including a coercivity H_(cJ)≧480 kA/m and aremanence B_(r)≧0.6 T.

A bonded magnet according to the present invention includes any of theiron-based rare-earth alloy nanocomposite magnet powders describedabove.

An inventive method of making an iron-based rare-earth alloynanocomposite magnet powder includes the step of obtaining a rare-earthalloy magnet powder by rapidly cooling a melt of an alloy by anatomization process. The alloy has a composition represented by thegeneral formula: (Fe_(1-m)T_(m))_(100-x-y-z)Q_(x)R_(y)Ti_(z)M_(n) (whereT is at least one element selected from the group consisting of Co andNi; Q is at least one element selected from the group consisting of Band C; R is at least one element selected from the group consisting ofthe rare-earth metal elements and yttrium; and M is at least one elementselected from the group consisting of Nb, Zr, Mo, Ta and Hf). The molefractions x, y, z, m and n satisfy the inequalities of: 10 at %<x≦25 at%, 6 at %≦y<10 at %, 0.1 at %≦z≦12 at %, 0≦m≦0.5, and 0 at %≦n≦10 at %,respectively. The method further includes the step of thermally treatingthe magnet powder, thereby forming a structure that includes at leasttwo types of ferromagnetic crystalline phases, in which a hard magneticphase has an average grain size of 10 nm to 200 nm and a soft magneticphase has an average grain size of 1 nm to 100 nm.

In one preferred embodiment, the step of rapidly cooling the melt by theatomization process results in making a rapidly solidified alloyincluding at least 60 vol % of R₂Fe₁₄B type compound phase.

In another preferred embodiment, the soft magnetic phase includes aferromagnetic iron-based boride.

In another preferred embodiment, the iron-based boride includes Fe₃Band/or Fe₂₃B₆.

In a preferred embodiment, an inventive method for producing a bondedmagnet includes the steps of: preparing a magnet powder by any of themethods described above; and processing the magnet powder into thebonded magnet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a view showing the configuration of a gas atomizationsystem for use in an embodiment of the present invention, and FIG. 1( b)is a perspective view illustrating a gas nozzle for use in this system.

FIG. 2 is a graph showing a demagnetization curve (i.e., the secondquadrant of a hysteresis curve) for an example of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to the present invention, an iron-based rare-earth alloynanocomposite magnet powder is obtained by rapidly cooling a melt of analloy, having a composition represented by the general formula:(Fe_(1-m)T_(m))_(100-x-y-z)Q_(x)R_(y)Ti_(z)M_(n) by an atomizationprocess.

In this general formula, T is at least one element selected from thegroup consisting of Co and Ni, Q is at least one element selected fromthe group consisting of B and C, R is at least one element selected fromthe group consisting of the rare-earth metal elements and yttrium, and Mis at least one element selected from the group consisting of Nb, Zr,Mo, Ta and Hf. The mole fractions x, y, z, m and n satisfy theinequalities of: 10 at %<x≦25 at %, 6 at %≦y<10 at %, 0.1 at %≦z≦12 at%, 0≦m≦0.5, and 0 at %≦n≦10 at %, respectively.

A 2-14-1 type hard magnetic compound, represented by the general formulaR₂(Fe_(1-m)T_(m))₁₄Q, will be simply referred to herein as an “R₂Fe₁₄Bbased compound (or R₂Fe₁₄B phase)”. That is to say, the “R₂Fe₁₄B basedcompound” or the “R₂Fe₁₄B phase” includes an R₂Fe₁₄B compound or phasein which a portion of Fe is replaced with Co and/or Ni or in which aportion of B is replaced with C. It should be noted that the “R₂Fe₁₄Bbased compound” may further include additive elements such as Ti and M.

The present inventors carried out extensive researches to make ananocomposite magnet by an atomization process. As a result, the presentinventors discovered that the nucleation and growth of the α-Fe phasewhile the melt was rapidly quenched could be minimized by adding Ti tothe material alloy. The present inventors acquired the basic idea of thepresent invention from this discovery.

According to the present invention, even at the relatively low quenchingrate achievable by the atomization process, the R₂Fe₁₄B phase as a mainphase can be nucleated earlier than the α-Fe phase and yet the size ofthe R₂Fe₁₄B phase is not allowed to reach such a size as to producemultiple magnetic domains in the R₂Fe₁₄B phase (i.e., 300 nm). As aresult, a magnet powder exhibiting excellent magnetic properties as ananocomposite magnet can be obtained.

An α-Fe/R₂Fe₁₄B based nanocomposite magnet to which the presentinvention is applied has a higher coercivity than an Fe₃B/Nd₂Fe₁₄B basednanocomposite magnet. However, the former nanocomposite magnet has arelatively low B mole fraction of about 7 at % to about 10 at % and arelatively high Nd mole fraction of about 10 at % to about 12 at %.Thus, its alloy has very low ability to from amorphous phases.Accordingly, if the atomization process is applied without adding any Tithereto, then just a powder in which the soft magnetic α-Fe phase hasgrown coarsely can be obtained and the magnetic properties thereof areinsufficient. On the other hand, the Fe₃B/Nd₂Fe₁₄B based nanocompositemagnet has a relatively high B mole fraction of about 18 at % to about20 at % and has high ability to form amorphous phases. However, themagnet has a relatively low Nd mole fraction of about 3 at % to about 5at % and exhibits low coercivity. Also, if the Fe₃B/Nd₂Fe₁₄B basednanocomposite magnet is produced by the atomization process, powderparticles with a relatively fine structure and amorphous phases can beobtained due to its high amorphous forming ability. Nevertheless, if themagnet is thermally treated and crystallized after that, the magnetitself generates some heat due to the crystallization and its crystalgrowth advances non-uniformly. Thus, the resultant nanocomposite magnetcannot exhibit high magnet performance.

As used herein, the “Fe₃B” includes “Fe_(3.5)B” which is hard todistinguish from “Fe₃B”.

According to the present invention, by defining the mole fractions of Tiand other elements within appropriate ranges, a nanocomposite magnetpowder with a nanocrystalline structure (i.e., a structure including atleast 60 vol % of Nd₂Fe₁₄B phase) can be obtained even by theatomization process. Thus, the heat treatment process forcrystallization does not have to be carried out after that. However, toincrease the uniformity of the alloy structure and further improve themagnet performance, the crystalline atomized powder is preferablythermally treated.

The structure of magnet powder particles according to the presentinvention includes at least two ferromagnetic crystalline phases, ofwhich the hard magnetic phase has an average size of 10 nm to 200 nm andthe soft magnetic phase has an average size of 1 nm to 100 nm. Moreparticularly, the R₂Fe₁₄B based compound phase exhibiting hard magneticproperties is uniformly dispersed in the alloy and the soft magneticboride and α-Fe phases are present on the grain boundary or sub-boundarythereof. The average crystal grain size of the α-Fe and boride phases issmaller than that of the R₂Fe₁₄B based compound phase, and is typically1 nm to 50 nm. In a preferred embodiment, iron-based borides (such asFe₃B and/or Fe₂₃B₆) with ferromagnetic properties nucleate as the boridephases and high magnetization can be maintained.

As the present inventors disclosed in Japanese Patent Application No.2001-144076, the boride phase (i.e., iron-based boride) is believed tobe present at least partially as a film on the grain boundary of theR₂Fe₁₄B based compound crystalline phase. When such a texture structureis obtained, the magnetic coupling (i.e., exchange interactions) betweenthe ferromagnetic boride phase and the main phase further consolidatesand the magnetization of the ferromagnetic boride phase is firmlymaintained. As a result, even higher coercivity is believed to beachieved.

The magnet powder of the present invention having such a texturestructure can be used effectively to make a bonded magnet. In making abonded magnet, the magnet powder preferably has a mean particle size of1 μm to 100 μm as measured by a scanning electron microscope.

Atomization Process

To make a permanent magnet powder by an atomization process from amolten alloy having the composition described above, a gas atomizationprocess, a centrifugal atomization process, a rotational electrodeprocess, a vacuum process, an impact process or any other suitableprocess may be adopted. When the centrifugal atomization process or therotational electrode process is adopted, the quenching rate ispreferably increased by blowing a gas at a high pressure.

Hereinafter, an embodiment that adopts a gas atomization process will bedescribed with reference to FIGS. 1( a) and 1(b).

FIG. 1( a) shows an exemplary configuration for a gas atomization systemto be preferably used in this embodiment. The system shown in FIG. 1( a)includes: a melting vessel 2 to melt an alloy by a high frequencyheating or resistance heating process and store the resultant moltenalloy 1 therein; and a spray chamber 4 in which a magnet powder (oratomized powder) 3 is formed by a gas atomization process. The meltingchamber, in which the melting vessel 2 is provided, and the spraychamber 4 are preferably filled with an inert atmosphere (of argon orhelium).

At the bottom of the melting vessel 2, a melt nozzle (with a nozzleorifice diameter of 0.5 mm to 3.0 mm) 5 is provided such that the moltenalloy 1 is ejected through the melt nozzle 5 into the spray chamber 2. Aringlike gas nozzle 6 such as that shown in FIG. 1( b) is provided underthe melt nozzle 5. A cooling gas is ejected strongly toward the centerof the ring through a plurality of holes of this ringlike gas nozzle 6.As a result, a great number of small droplets of the molten alloy areformed and rapidly quenched while being deprived of the heat by thesurrounding gas. Then the rapidly quenched and solidified metal dropletsare collected as the magnet powder 3 at the bottom of the gasatomization system.

When such a gas atomization system is used, the particle sizedistribution of the powder can be controlled by adjusting the viscosityof the molten alloy and the energy of the spray gas.

It should be noted that when a molten alloy having a poor ability toform amorphous phases is rapidly quenched and solidified by a gasatomization process, powder particles with an amorphous ornanocrystalline structure cannot be obtained unless the atomizationprocess is carried out under such conditions as to form powder particleswith particle sizes of 20 μm or less, for example. This is because thesmaller the particle sizes of the powder particles to be obtained, thegreater the ratio of the surface area to the volume of the respectiveparticles and the higher the cooling effects. In the prior art, thegreater the particle size, the lower the quenching rate of insideportions of particles. As a result, a crystal structure with anexcessively large size is formed and the resultant magnet performancedeteriorates. When such a phenomenon occurs, the magnet performancedeteriorates significantly in a nanocomposite magnet powder, inparticular.

In contrast, according to the present invention, even if the powderparticle sizes are as large as 20 μm to 100 μm, the inside portions ofthe powder particles can also be rapidly quenched uniformly and at asufficiently high rate. Thus, a nanocomposite magnet powder exhibitingexcellent magnetic properties can be obtained.

Heat Treatment

Thereafter, the magnet powder, obtained by using the gas atomizationsystem described above, is preferably thermally treated within an inertatmosphere of argon (Ar), for example. The temperature increase rate ofthe heat treatment process is preferably 0.08° C./s to 15° C./s.Specifically, the magnet powder is preferably maintained at atemperature of 500° C. to 800° C. for a period of time of 30 seconds to60 minutes, and then cooled to room temperature. By carrying out thisheat treatment process, an almost completely crystalline structure canbe obtained even if some amorphous phases are left in the powderparticles as a result of the gas atomization process.

The heat treatment atmosphere is preferably an inert gas such as Ar gasor N₂ gas to minimize the oxidation of the alloy. Alternatively, theheat treatment may also be carried out within a vacuum of 1.3 kPa orless.

It should be noted that if carbon is added to the material alloy, theoxidation resistance of the magnet powder can be increased. If asufficient amount of carbon has been added to the material alloy, thenthe atomized powder may be heat-treated in the air. Also, the magnetpowder of this embodiment already has a spherical shape whencrystallized by the atomization process, and is not subjected to anymechanical pulverization process thereafter. Accordingly, the surfacearea of the powder per unit mass is far smaller than that of a knownmechanically pulverized powder. Thus, the magnet powder is notoxidizable so easily even when exposed to the air during the heattreatment process or any other process.

In making a bonded magnet from this magnet powder, the magnet powder iscompounded with an epoxy resin or a nylon resin and then molded into adesired shape. In this case, a magnet powder of any other type (e.g., arapidly solidified alloy type magnet powder other than nanocompositemagnets, an Sm-T-N based magnet powder and/or hard ferrite magnetpowder) may be mixed with the magnet powder of the present invention.

Why this Alloy Composition is Preferred

Q is either B (boron) only or a combination of B and C (carbon). If themole fraction x of Q is 10 at % or less, then the amorphous formingability will be too low to obtain the desired nanocrystalline structureeasily at a normal quenching rate (of about 10²° C./s to about 10⁴°C./s) for a gas atomization process. On the other hand, if the molefraction x of Q exceeds 25 at %, then the percentage of the α-Fe phase,which has a higher saturation magnetization than any other constituentphase, decreases excessively and the remanence B_(r) drops. In view ofthese considerations, the mole fraction x of Q is preferably adjustedsuch that 10 at %<x≦25 at %. A more preferable x range is 10 at %<x≦20at % and an even more preferable x range is 13 at %≦x≦18 at %.

C decreases the size of the metal structure in the rapidly solidifiedalloy, and therefore, plays an important role when the melt quenchingrate is as low as in the gas atomization process. Also, by adding C, thenucleation of TiB₂ is minimized, thus decreasing the solidificationstart temperature of the molten alloy. As a result, the teemingtemperature can be lowered. In that case, the alloy can be melted in ashorter time, and the quenching rate can be increased. However, if theatomic ratio of C to the total amount of Q exceeds 0.5, then the α-Fewill be produced noticeably, the constituent phases will change, and themagnetic properties will deteriorate. This is why the atomic ratio of Cto the total amount of Q is preferably 0.01 to 0.5. A more preferablerange is 0.05 to 0.3 and an even more preferable range is 0.08 to 0.20.

R is at least one element selected from the group consisting of therare-earth elements and yttrium. R is an indispensable element forR₂Fe₁₄B, which is a hard magnetic phase that cannot be omitted toachieve permanent magnet properties. Specifically, R preferably includesPr or Nd as an indispensable element, a portion of which may be replacedwith Dy and/or Tb. If the mole fraction y of R is less than 6 at %, thenthe compound phase having the R₂Fe₁₄B type crystal structure, whichcontribute to expressing coercivity, do not crystallize sufficiently anda coercivity H_(cJ) of 480 kA/m or more cannot be obtained. On the otherhand, if the mole fraction y of R is equal to or greater than 10 at %,then the percentages of the iron-based borides and α-Fe withferromagnetic properties both decrease. For these reasons, the molefraction y of the rare-earth element R is preferably 6 at %≦y<10 at %. Amore preferable range for R is 7 at %≦y≦9 at %, and an even morepreferable range for R is 7.5 at %≦y≦8.5 at %.

Ti is an indispensable element for a nanocomposite magnet according tothe present invention. By adding Ti, high hard magnetic properties areachieved, and the loop squareness of the demagnetization curve isimproved. As a result, the maximum energy product (BH)_(max) can also beincreased.

Unless Ti is added, the α-Fe easily nucleates and grows before theR₂Fe₁₄B phase nucleates and grows. Accordingly, when the atomized alloyis thermally treated and crystallized, the soft magnetic α-Fe phase willhave grown excessively.

In contrast, if Ti is added, the nucleation and growth kinetics of theα-Fe phase is slowed down, i.e., it would take a longer time for theα-Fe phase to nucleate and grow. Thus, the present inventors believethat the R₂Fe₁₄B phase starts to nucleate and grow before the α-Fe phasehas nucleated and grown. Thus, the R₂Fe₁₄B phase can be grownsufficiently and distributed uniformly before the α-Fe phase has growntoo much.

Also, Ti has a strong affinity for B and is easily condensed in aniron-based boride. Thus, the present inventors believe that Tistabilizes an iron-based boride by producing a strong bond to B in theiron-based boride.

Furthermore, by adding Ti, the viscosity of the molten alloy decreasesto 5×10⁻⁶ m²/s or less. As a result, a preferred melt viscosity for theatomization process is obtained.

If the mole fraction of Ti exceeds 12 at %, then the production of theα-Fe is reduced significantly, thus decreasing the remanence Brunintentionally. On the other hand, if the mole fraction of Ti is lowerthan 0.1 at %, then the R₂Fe₁₄B phase, which cannot be omitted toexpress the coercivity, will not crystallize sufficiently. Also, even ifthe R₂Fe₁₄B phase has crystallized sufficiently, the desired uniformnanocrystalline structure cannot be obtained, either, and highcoercivity cannot be achieved. In view of these considerations, apreferable z range is 0.1 at %≦z≦12 at %, and a more preferable z rangeis 1 at %≦z≦7 at %.

The element M is an element that increases the amorphous forming abilityof the alloy. If M is added, the amorphous forming ability of the alloyincreases and the growth of the crystalline phases can be minimizedwhile the molten alloy is being quenched. And a magnet with good loopsquareness in its demagnetization curve can be obtained as a result ofthe subsequent heat treatment process. In addition, by adding M, theintrinsic coercivity can also be increased.

If the mole fraction of M exceeds 10 at %, then the grain growth of therespective constituent phases advances remarkably in a relatively hightemperature range in which the α-Fe phase grows rapidly, the exchangecoupling among the respective constituent phases weakens, and the loopsquareness of the demagnetization curve decreases significantly. Forthat reason, the mole fraction n of the element M preferably satisfies 0at %≦n≦10 at %. A more preferable n range is 0.5 at %≦n≦8 at % and aneven more preferable n range is 1 at %≦n≦6 at %. It should be noted thatAl, Si, Mn, Cu, Ga, Ag, Pt, Au and/or Pb, as well as M, may also beadded as an element that reduces the size of the metal structure.

The balance of the material alloy, other than the elements describedabove, may be Fe alone. Alternatively, at least one transition metalelement T, selected from the group consisting of Co and Ni, may besubstituted for a portion of Fe, because the desired hard magneticproperties are achievable in that case also. However, if the percentagem of Fe replaced with T exceeds 50%, then a high remanence Br cannot beobtained. For that reason, the percentage m of Fe replaced is preferably50% or less (i.e., 0≦m≦0.5). A more preferable highest allowablereplacement percentage m is 40%. Also, by substituting Co for a portionof Fe, the hard magnetic properties improve and the Curie temperature ofthe R₂Fe₁₄B phase increases, thus increasing the thermal resistance.

Manufacturing Process

To make a permanent magnet powder by an atomization process from amolten alloy having the composition described above, a gas atomizationprocess, a centrifugal atomization process, a rotational electrodeprocess, a vacuum process, an impact process or any other suitableprocess may be adopted.

If a gas atomization process is adopted, the melt may be sprayed intothe inert gas through a nozzle with a nozzle orifice diameter of 0.5 mmto 3.0 mm. In this case, the melt may be sprayed on a high-velocityinert gas flow. This gas may have a pressure of 0.1 MPa to 7 MPa, forexample. According to the gas atomization method, the particle sizedistribution of the powder can be controlled by adjusting the viscosityof the molten alloy and the energy of the spray gas.

When a molten alloy having a poor ability to form amorphous phases israpidly quenched and solidified by a gas atomization process, powderparticles with an amorphous or nanocrystalline structure cannot beobtained unless the atomization process is carried out under suchconditions as to form powder particles with particle sizes of 20 μm orless, for example. This is because the smaller the particle sizes of thepowder particles to be obtained, the greater the ratio of the surfacearea to the volume of the respective particles and the higher thecooling effects. In the prior art, the greater the particle size, thelower the quenching rate of inside portions of particles. As a result, acrystal structure with an excessively large size is formed and theresultant magnet performance deteriorates. When such a phenomenonoccurs, the magnet performance deteriorates significantly in ananocomposite magnet powder, in particular.

In contrast, according to the present invention, even if the powderparticle sizes are as large as 20 μm to 100 μm, the inside portions ofthe powder particles can also be rapidly quenched uniformly and at asufficiently high rate. Thus, a nanocomposite magnet powder exhibitingexcellent magnetic properties can be obtained.

EXAMPLES

For each of Samples Nos. 1 through 4 having the compositions shown inthe following Table 1, respective materials thereof. Nd, Pr, Fe, Co, B,C, Ti, Nb, Zr, Si and Cu having purities of 99.5% or more were weighedso that the mixture had a total weight of 1 kg (kilogram). The mixturewas subjected to a gas atomization process under the followingconditions, thereby making a powder with a mean particle size of about50 μm. Thereafter, the powder particles were classified to obtain apowder with particle sizes of 63 μm or less.

Gas used: argon (Ar);

Gas pressure: 40 kgf/cm² (=3.92 MPa);

Spraying temperature: 1,400° C.; and

Melt feeding rate: 2.0 kg/min.

TABLE 1 Heat Treat- ment Alloy Temp- composition (at %) erature No. R TQ Ti M Other (° C.) EX- 1 Nd9 Fe as B12.6 + 3 Nb1 — 740 AM- bal- C1.4PLES ance 2 Nd7.4 Fe as B11 + 3 Nb1 Si0.5 720 bal- C2 ance 3 Nd7.6 Co5B12.5 + 4 Nb0.3 + Cu0.2 740 + C2.5 Zr0.1 Fe as bal- ance 4 Nd8.4 + Fe asB12 + 1 Zr2.6 — 720 Pr0.5 bal- C1 ance

In Table 1, “Nd7.8+Pr0.6” included in the column “R” indicates that 7.8at % of Nd and 0.6 at % of Pr were added, and “B12+C1” included in thecolumn “Q” indicates that 12 at % of B (boron) and 1 at % of C (carbon)were added.

Next, this atomized powder was heated to, and maintained at, any of theheat treatment temperatures shown in Table 1 for five minutes within anAr gas atmosphere and then cooled to room temperature. The magneticproperties of the thermally treated atomized powder were measured with avibrating sample magnetometer (VSM). The results obtained from powderswith particle sizes of 25 μm or less are shown in the following Table 2and the demagnetization curve of the thermally treated atomized powderis shown in FIG. 1.

TABLE 2 B_(r) (T) H_(cJ) (kA/m) (BH)_(max) (kJ/m³) EXAMPLES 1 0.74 90181.2 2 0.79 605 84 3 0.77 629 79.6 4 0.70 933 75.8

As can be seen, a nanocomposite magnet powder exhibiting hard magneticproperties including a coercivity H_(cJ)≧480 kA/m and a remanenceB_(r)≧0.6 T can be obtained according to this example.

According to the present invention, even though an atomization process,which results in a lower quenching rate than a melt spinning process orany other rapid quenching process, is adopted, the R₂Fe₁₄B phase can benucleated earlier than the α-Fe phase by optimizing the alloycomposition and adding Ti thereto. As a result, a magnet powder with ananocrystalline structure that can exhibit excellent magnetic propertiesas a nanocomposite magnet can be obtained. In addition, according to thepresent invention, spherical particles for an alloy magnet can bedirectly produced by the atomization process. Thus, no pulverizingprocess step is needed, and the manufacturing cost of magnets can bereduced significantly.

1. A nanocomposite magnet powder of an iron-based rare-earth alloyhaving a composition represented by the general formula:(Fe_(1-m)T_(m))_(100-x-y-z-n)Q_(x)R_(y)Ti_(z)M_(n) (where T is at leastone element selected from the group consisting of Co and Ni; Q is atleast one element selected from the group consisting of B and C; R is atleast one element selected from the group consisting of the rare-earthmetal elements and yttrium; and M is at least one element selected fromthe group consisting of Nb, Zr, Mo, Ta and Hf), the mole fractions x, y,z, m and n satisfying the inequalities of: 10 at %<x≦25 at %; 6 at%≦y<10 at %; 0.1 at %≦z≦12 at %; 0≦m≦0.5; and 0 at %≦n≦10 at %,respectively, wherein the nanocomposite magnet powder is obtained by anatomization process and includes at least two types of ferromagneticcrystalline phases, in which an R₂Fe₁₄B type compound phase serving as ahard magnetic phase has an average grain size of 10 nm to 200 nm and asoft magnetic phase has an average grain size of 1 nm to 50 nm; aferromagnetic iron-based boride phase and an α-Fe phase serving as thesoft magnetic phase are present on a grain boundary or sub-boundary ofthe R₂Fe₁₄B type compound phase; and the average crystal grain size ofthe ferromagnetic iron-based boride phase and α-Fe is smaller than thatof the R₂Fe₁₄B type compound phase.
 2. The iron-based rare-earth alloynanocomposite magnet powder of claim 1, wherein the R₂Fe₁₄B typecompound phase, the boride phase and the α-Fe phase are present in thesame metal structure.
 3. The iron-based rare-earth alloy nanocompositemagnet powder of claim 1, wherein the iron-based boride includes Fe₃Band/or Fe₂₃B₆.
 4. The iron-based rare-earth alloy nanocomposite magnetpowder of claim 1, wherein the magnet powder has a mean particle size of1 μm to 100 μm.
 5. The iron-based rare-earth alloy nanocomposite magnetpowder of claim 1, wherein the magnet powder exhibits hard magneticproperties including a coercivity H_(cj)≧480 kA/m and a remanenceB_(r)≧0.6 T.
 6. A bonded magnet comprising the iron-based rare-earthalloy nanocomposite magnet powder of claim
 1. 7. A method of making aniron-based rare-earth alloy nanocomposite magnet powder, the methodcomprising the steps of: obtaining a rare-earth alloy magnet powder byrapidly cooling a melt of an alloy by an atomization process, the alloyhaving a composition represented by the general formula:(Fe_(1-m)T_(m))_(100-x-y-z-n)Q_(x)R_(y)Ti_(z)M_(n) (where T is at leastone element selected from the group consisting of Co and Ni; Q is atleast one element selected from the group consisting of B and C; R is atleast one element selected from the group consisting of the rare-earthmetal elements and yttrium; and M is at least one element selected fromthe group consisting of Nb, Zr, Mo, Ta and Hf), the mole fractions x, y,z, m and n satisfying the inequalities of: 10 at %<x≦25 at %; 6 at%≦y<10 at %; 0.1 at %≦z≦12 at %; 0≦m≦0.5, and 0 at %≦n≦10 at %,respectively; and thermally treating the magnet powder, thereby forminga structure that includes at least two types of ferromagneticcrystalline phases, in which an R₂Fe₁₄B type compound phase serving as ahard magnetic phase has an average grain size of 10 nm to 200 nm and asoft magnetic phase has an average grain size of 1 nm to 50 nm; whereina ferromagnetic iron-based boride phase and an α-Fe phase serving as thesoft magnetic phase are present on a grain boundary or sub-boundary ofthe R₂Fe₁₄B type compound phase; and the average crystal grain size ofthe ferromagnetic iron-based boride phase and α-Fe is smaller than thatof the R₂Fe₁₄B type compound phase.
 8. The method of claim 7, whereinthe step of rapidly cooling the melt by the atomization process resultsin making a rapidly solidified alloy including at least 60 vol % of theR₂Fe₁₄B type compound phase.
 9. The method of claim 7, wherein theiron-based boride includes Fe₃B and/or Fe₂₃B6.
 10. A method forproducing a bonded magnet, the method comprising the steps of: preparingan iron-based rare-earth alloy nanocomposite magnet powder by the methodclaim 7, and processing the magnet powder into the bonded magnet.